The complex variable soft X-ray spectrum of NGC 5548
Abstract
We present ROSAT 0.2-2 keV monitoring data on NGC 5548 taken over the period 1992 December-1993 January, when the source was at its historical maximum in X-ray flux. The data show a soft excess and partially ionized absorber modifying the high-energy power-law (alpha~0.9) spectrum. The major variability during the campaign is a strong flare event over 8d, in which the soft excess brightens without a corresponding change in the power-law intensity. This is incompatible with the simple reprocessing models which are currently popular, where the soft excess is produced from thermalized hard X-ray emission illuminating an accretion disc. Given the high source luminosity, >=0.1 L_Edd, it seems likely that the soft excess is dominated by intrinsic emission from the accreting material. While the data do not allow the ionization of the absorbing material to be constrained in each single spectrum, a comparison of the brightest spectrum with that from a long exposure taken 2d later and which is a factor of 2 fainter shows a significant decrease in the ionization. This implies a density of >=5x10^5 cm^-3 for the warm material, and hence a distance of less than ~10^18 cm from the central source.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- July 1995
- DOI:
- 10.1093/mnras/275.2.417
- Bibcode:
- 1995MNRAS.275..417D
- Keywords:
-
- ACCRETION;
- ACCRETION DISCS;
- GALAXIES: ACTIVE;
- GALAXIES: INDIVIDUAL: NGC 5548;
- X-RAYS: GALAXIES